metamorphism of late neoproterozoic-early cambrian schists ... · intermediate pressure barrovian...

Journal of Sciences, Islamic Republic of Iran 23(2): 147-161 (2012) http://jsciences.ut.ac.ir University of Tehran, ISSN 1016-1104

147

Metamorphism of Late Neoproterozoic-Early Cambrian

Schists in Southwest of Zanjan from the Soltanieh

Belt in Northwest of Iran

A. Ghadimi, J. Izadyar,* S. Azimi, M. Mousavizadeh,

and M. Eram

Department of Geology, Faculty of Science, Zanjan University Zanjan, Islamic Republic of Iran

Received: 6 February 2011 / Revised: 14 May 2012 / Accepted: 30 July 2012

Abstract

Late Neoproterozoic-Early Cambrian schists have been occurred in southwest

of Zanjan city from the Soltanieh belt. The Soltanieh belt in northwest of Iran is

uplifted basement of Precambrian-Paleozoic in main central Iran zone and

includes outcrops of Precambrian, Paleozoic and Mesozoic Formations. Late

Neoproterozoic-Early Cambrian schists, the oldest stratigraphy unit in the region,

consist of phyllite, chlorite schist, mica schist and staurolite schist. Field and

microstructural studies show several deformational phases in which the first phase

(D1) is well characterized by S1 schistosity subparallel to the axial planes of F1

fold. D2 is a post D1-phase of deformation recognized by kink-like mesoscopic

folds that fold S1 and the third phase (D3) is characterized by strongly developed

crenulation cleavage (S3). Considering textural relationship, microstructural

domain and mineral chemistry two main metamorphic events (M1 and M2) have

been recognized. Geothermobarometry of the M2 assemblage containing white

mica, chlorite, biotite, plagioclase and quartz in the KNCMFASH system

considering water saturated condition, yields pressure of 6.3Kbar and temperature

of 690°C and the P-T estimates for the M1 assemblage containing white mica,

chlorite, biotite, plagioclase, staurolite and quartz yield pressure of 8.5Kbar and

temperature of 580°C. The obtained clockwise P-T path shows an early

intermediate pressure Barrovian type metamorphism (M1) following by a low

pressure Buchan type metamorphism (M2). The deduced clockwise P-T path is

characteristic of the metamorphic evolution of orogenic belts and probably

originated through the convergence of the Gondwana and Eurasian plates during

the Pan-African orogeny.

Keywords: Late Neoproterozoic-Early Cambrian schists; Soltanieh belt; Zanjan; Iran;

Geothermobarometry

* Corresponding author, Tel.: +98(241)5154030, Fax: +98(241)5154002, E-mail: [email protected]

Introduction

Iranian crust constitutes one of the largest tectonic

provinces in the southwestern Asia and is regarded as a

complicated puzzle of continental fragments initially

rifted from Gondwana which presently separated from

each other by complex fold and thrust and discontinuous

ophiolitic belts [5, 29]. The major tectonic provinces of

Vol. 23 No. 2 Spring 2012

western and central Iran from north to south include: the

Alborz mountains, a variety of small blocks composing

central Iran, the Urmieh–Dokhtar arc, the Sanandaj

Sirjan zone, the Zagros mountains and the Zagros

foreland (Arabian shield) [14] (Fig. 1).

tectonic provinces are separated by two major suture

zones representing closure of Paleotethys and Neotethys

oceans. Paleotethys ocean closed during collision of

Iran and Eurasia in the Late Triassic and Neotethys was

closed in the Late Cretaceous or Tertiary because of

collision of Arabia with Iran [3, 32, 33

Historically, intensely metamorphosed rocks

(migmatite, gneiss, amphibolite and micaschist) that

crop out in the major tectonic provinces of Iran (such as

central Iran, Sanandaj-Sirjan, Alborz) visualized to

present the Precambrian basement of the Iranian plateau

[19, 34] and viewed as analogous of the Precambrian

basement rocks exposed in the Arabian shield (e.g.

[34]). It has been widely stated that the consolidation of

the basement of Iranian plateau took place by the

metamorphism and anatexis at about 11000

[35,5,7]. However, earlier attempts at dating

metamorphic rocks was controversial from Archean

[13] to Early Proterozoic [27]. The earliest modern

zircon U-Pb data include those of Samani et al

the Saghand (east central Iran) and Ozbak kuh (northern

Lut block) area. Their U-Pb ages are mostly discordant

and were interpreted to indicate magmatic and

metasomatic events from Middle to Late Proterozoic.

Ramezani [24] and Ramezani and Tucker

out the first extensive research on Iran basement

complexes employing TIMS U-Pb zircon dating of a

correlative suite of rocks in central Iran.

mentioned that the oldest documented and isotopically

dated rocks in the Saghand region and probably the

entire area of central Iran belong to the Tashk

Formation with the depositional age between

and 533Ma. The well stratified sequence of weakly

metamorphosed, sedimentary and volcanic/

volcaniclastic rocks that occur in vast area of Alborz,

Zanjan, Azarbaijan is known as the Kahar Formation

[8]. Crawford [6] was the first researcher who used Rb

Sr dating method for the Kahar Formation and obtained

645 Ma age. Horton et al. [17], using U

determination suggested a sediment source during Late

Neoproterozoic-Early Cambrian sedimentation and

proposed that deposition of the Kahar Formation was

completed by the end of Early Cambrian in agreement

with the Middle to Late Cambrian age assigned for

overlying fossiliferous carbonate [11]. These data are

good agreement with a depositional age constrained

between 627 Ma and 533 Ma for the Tashk Formation

in central Iran, a lateral equivalent of the Kahar

Ghadimi et al.

148

south include: the

Alborz mountains, a variety of small blocks composing

Dokhtar arc, the Sanandaj–

Sirjan zone, the Zagros mountains and the Zagros

). The major

by two major suture

zones representing closure of Paleotethys and Neotethys

Paleotethys ocean closed during collision of

Iran and Eurasia in the Late Triassic and Neotethys was

closed in the Late Cretaceous or Tertiary because of

33] (Fig. 1).

Historically, intensely metamorphosed rocks

(migmatite, gneiss, amphibolite and micaschist) that

crop out in the major tectonic provinces of Iran (such as

Sirjan, Alborz) visualized to

recambrian basement of the Iranian plateau

and viewed as analogous of the Precambrian

basement rocks exposed in the Arabian shield (e.g.

It has been widely stated that the consolidation of

the basement of Iranian plateau took place by the

11000 to 600 Ma

However, earlier attempts at dating

metamorphic rocks was controversial from Archean

The earliest modern

Pb data include those of Samani et al. [26] for

e Saghand (east central Iran) and Ozbak kuh (northern

Pb ages are mostly discordant

and were interpreted to indicate magmatic and

metasomatic events from Middle to Late Proterozoic.

and Ramezani and Tucker [25] carried

out the first extensive research on Iran basement

Pb zircon dating of a

correlative suite of rocks in central Iran. They

mentioned that the oldest documented and isotopically

dated rocks in the Saghand region and probably the

ntire area of central Iran belong to the Tashk

Formation with the depositional age between 627Ma

The well stratified sequence of weakly

metamorphosed, sedimentary and volcanic/

volcaniclastic rocks that occur in vast area of Alborz,

baijan is known as the Kahar Formation

was the first researcher who used Rb-

for the Kahar Formation and obtained

using U-Pb age

determination suggested a sediment source during Late

Early Cambrian sedimentation and

proposed that deposition of the Kahar Formation was

completed by the end of Early Cambrian in agreement

with the Middle to Late Cambrian age assigned for

These data are in

good agreement with a depositional age constrained

Ma for the Tashk Formation

in central Iran, a lateral equivalent of the Kahar

Formation [25]. Also, the possible correlatives for the

Kahar Formation are successions including Mo

series [18], Kalmard Formation [35

Formation [23]. These imply that Kahar type facies is

widely distributed throughout the Iranian plateau is

likely underlie Phanerozic platform strata in many

locations [25]. Although, these rocks are

elucidate the geodynamic evolution of the Iranian

plateau, but their metamorphic history have not hitherto

been studied in detail. The studied metamorphic rocks

are part of the Kahar Formation [36,

best opportunity to widen the existent knowledge of

such rocks. Thus, the aims of this paper are: to present

metamorphic data of Late Neoproterozoic

Cambrian schsits and also to clarify the geodynamic

evolution of the region and to investigate the possible

implications for metamorphic evolution of Kahar type

rocks in the Iranian plateau.

Materials and Methods

1-Geological Setting

The Soltanieh belt forms a long and narrow

Figure 1. Major sedimentary and structural units of

Iran (simplified from Aghanabati

J. Sci. I. R. Iran

Also, the possible correlatives for the

Kahar Formation are successions including Morad

35] and upper Taknar

These imply that Kahar type facies is

widely distributed throughout the Iranian plateau is

likely underlie Phanerozic platform strata in many

Although, these rocks are important to

elucidate the geodynamic evolution of the Iranian

plateau, but their metamorphic history have not hitherto

The studied metamorphic rocks

, 4] and provides the

n the existent knowledge of

Thus, the aims of this paper are: to present

metamorphic data of Late Neoproterozoic-Early

Cambrian schsits and also to clarify the geodynamic

evolution of the region and to investigate the possible

metamorphic evolution of Kahar type

Materials and Methods

The Soltanieh belt forms a long and narrow

Major sedimentary and structural units of

simplified from Aghanabati [1]).

Metamorphism of Late Neoproterozoic

Figure 2. Geological map of the Soltanieh area

northwest-southeast belt with more than 150

and a maximum of 10 to 12 Km width

Soltanieh belt, in fact, is the uplifted basement of

Precambrian-Paleozoic in main central Iran zone and

maintains Precambrian, Paleozoic and even Mesozoic

Formations completely similar to Alborz zone

indicating that from Precambrian until the end of

Jurassic, they formed one sedimentary basin

narrowness and the straight alignment of the uplift

indicate it's nature as a major fault zone clearly

expressed by a longitudinal fault along it’s strike in

northeast border (Fig. 2). Along the Soltanieh fault, the

pre-Tertiary Formations of the Soltanieh belt are faulted

against and partly thrust over the Tertiary Formations

which fill the Zanjan-Abhar depression

contrast to the faulted northeast border the southwest

border in spite of its straight alignment gives no surface

evidence of faulting. The straight alignment of the

southwestern mountain border may thus be explained as

a surface expression of persistent line of down

below the southwest plain of Kavand-Do Tappeh (Fig.

2). In additional to the bordering thrust fault in the

northeast and border flexture in the southwest, both

having straight northwest-southeast trend, the

Metamorphism of Late Neoproterozoic-Early Cambrian Schists in Southwest of Zanjan from

149

Geological map of the Soltanieh area (simplified from Stöcklin and Eftekharnezhad

150 Km length

Km width [36]. The

Soltanieh belt, in fact, is the uplifted basement of

Paleozoic in main central Iran zone and

maintains Precambrian, Paleozoic and even Mesozoic

y similar to Alborz zone

indicating that from Precambrian until the end of

they formed one sedimentary basin [4]. This

narrowness and the straight alignment of the uplift

indicate it's nature as a major fault zone clearly

nal fault along it’s strike in

Along the Soltanieh fault, the

Tertiary Formations of the Soltanieh belt are faulted

against and partly thrust over the Tertiary Formations

Abhar depression (Fig. 2). In

ntrast to the faulted northeast border the southwest

border in spite of its straight alignment gives no surface

The straight alignment of the

southwestern mountain border may thus be explained as

e of down-flexuring

Do Tappeh (Fig.

In additional to the bordering thrust fault in the

northeast and border flexture in the southwest, both

southeast trend, the Soltanieh

mountains display a great number of cross faults of

various directions that disrupt the pre

Formations into a complicated mosaic

pattern [36, 4] (Fig. 2). Complete sequence of detritic,

pyroclastic and carbonate thick deposits (Kahar,

Bayandor, Soltanieh, Barout and Lalun Formations) are

present in the study area which are important for the

geological analysis of the Iranian crust

3). Interestingly, the type localities of Bayandor,

Soltanieh and Barout Formations have been sel

from within the study area [36].

present TIMS zircon U-Pb results for the mentioned

sequence. Their zircon U-Pb data show that earliest

sediment accumulation commenced in the Latest

Neoproterozoic prior to the Precambrian

boundary. The Late Neoproterozoic-

of Kahar, Bayandor, Soltanieh, Barout and Lalun

Formations was also assigned by other researchers (e.g.

[34, 35, 5, 27, 2]). Considering these studies, in this

paper, this sequence was also considered as

Neoproterozoice-Early Cambrian deposits.

southwest of Zanjan and in contact with Doran granite

from south of Chavarzagh village towards north of

Rayhan village, a metamorphic complex with NW

Early Cambrian Schists in Southwest of Zanjan from…

simplified from Stöcklin and Eftekharnezhad [36]).

play a great number of cross faults of

various directions that disrupt the pre-Tertiary

Formations into a complicated mosaic-like fault block

Complete sequence of detritic,

pyroclastic and carbonate thick deposits (Kahar,

, Soltanieh, Barout and Lalun Formations) are

present in the study area which are important for the

geological analysis of the Iranian crust [36] (Figs. 2 and

Interestingly, the type localities of Bayandor,

Soltanieh and Barout Formations have been selected

Horton et al. [17]

Pb results for the mentioned

Pb data show that earliest

sediment accumulation commenced in the Latest

Neoproterozoic prior to the Precambrian-Cambrian

-Early Cambrian age

of Kahar, Bayandor, Soltanieh, Barout and Lalun

Formations was also assigned by other researchers (e.g.

Considering these studies, in this

paper, this sequence was also considered as Late

Early Cambrian deposits. From the

southwest of Zanjan and in contact with Doran granite

from south of Chavarzagh village towards north of

Rayhan village, a metamorphic complex with NW-SE


Figure 3. Geological map of the studied area

trend has been outcropped (Fig. 3). It consists of

phyllite, chlorite schist, mica schist and staurolite schist

with lenses and layers of quartzite and talc schist. They

have been considered as the oldest stratigraphic unit in

the area [36, 4]. The metamorphic complex cannot be

sharply separated from the barely metamorphosed slate

sequences constituting the Kahar Formation but seems

to be linked with them by both vertical and lateral

transitions [36]. Thus, it is possible to consider them as

a lateral equivalent of the Kahar Formation which differ

because of different metamorphic grade so that from

Chavarzagh village towards Rayhan village, metamor

phic mineral zones including chlorite, biotite and

staurolite can be determined. In the study area,

Ordovician, Silurian and Devonian deposits have not

been observed and the Permian deposits (Dorud and

Ruteh Formations) overly uncomformably the Milla

Formation (Figs. 2 and 3). Such observation was also

reported by other researchers (e.g. [36, 4

Ghadimi et al.

150

Geological map of the studied area (simplified from Babakhani and Sadeghi [4]).

It consists of

phyllite, chlorite schist, mica schist and staurolite schist

ite and talc schist. They

have been considered as the oldest stratigraphic unit in

The metamorphic complex cannot be

sharply separated from the barely metamorphosed slate

sequences constituting the Kahar Formation but seems

ith them by both vertical and lateral

Thus, it is possible to consider them as

Kahar Formation which differ

because of different metamorphic grade so that from

Chavarzagh village towards Rayhan village, metamor-

phic mineral zones including chlorite, biotite and

staurolite can be determined. In the study area,

Ordovician, Silurian and Devonian deposits have not

been observed and the Permian deposits (Dorud and

uncomformably the Milla

Such observation was also

4]). Mesozoic

deposits contain detrital deposits of the Shemshak

Formation and limestones of the Lar Formation

and 3). The Shemshak Formation deposits includ

sequence of shale with sandstone interlayers and

occasionally thin lenticular layers of coal. On the basis

of fossil plants, the Rhaeto-Liassic age has been

considered for the Shemshak Formation

Soltanieh mountain, the soft-weathering s

sandstones of the Shemshak Formation are conformably

overlain by limestone of the Lar Formation.

deposits consist of a sequence of detrital, pyroclastic

and volcanic rocks which are equivalent to the Karaj

Formation in Alborz. The overla

deposits on older Formations is with unconformity

everywhere, which begins with Fajan conglomerate

(Figs. 2 and 3). In the study area, the contact between

the Fajan and older units is faulted.

appears to be supported by the fact that Fajan

conglomerate do not contain any clasts of the

J. Sci. I. R. Iran

]).

deposits contain detrital deposits of the Shemshak

Formation and limestones of the Lar Formation (Figs. 2

The Shemshak Formation deposits include a

sequence of shale with sandstone interlayers and

occasionally thin lenticular layers of coal. On the basis

Liassic age has been

considered for the Shemshak Formation [36, 4]. In the

weathering shales and

sandstones of the Shemshak Formation are conformably

overlain by limestone of the Lar Formation. Tertiary

deposits consist of a sequence of detrital, pyroclastic

and volcanic rocks which are equivalent to the Karaj

The overlap of the Tertiary

deposits on older Formations is with unconformity

which begins with Fajan conglomerate [36]

In the study area, the contact between

Fajan and older units is faulted. This observation

by the fact that Fajan

conglomerate do not contain any clasts of the

Metamorphism of Late Neoproterozoic-Early Cambrian Schists in Southwest of Zanjan from…

151

metamorphic complexes. This observation was also

supported by other researchers (e.g. [14]). A small body

of deeply eroded leucogrnite is partially exposed in the

low topography around the village of Doran, about 12

Km south of Zanjan. Stöcklin and Eftekharnezhad [36]

mapped this intrusion as the Doran granite (Figs. 2 and

3). The leucogranite contact with the Bayandor and

Fajan Formations is faulted [14] but it is intruded into

the Kahar Formation and the metamorphic complex and

locally caused generation of biotite hornfels in the

contact aureole. Within Doran leucogranite, large

Figure 4. Photograph of biotite-chlorite-muscovite schist

showing a) F1 and F2 folds b) F1 and F2

folds c) F3 fold.

enclaves of slate and phyllite of the Kahar Formation

and metamorphic complex could be seen. Stöcklin and

Eftekharnezhad [36] asserted that the siliciclastic

Neoproterozoic Bayandor Formation unconformably

overlies the eroded and saprolitic surface of the granite.

Based on this relationship, Doran granite has been

considered in the literature as the type example of

Precambrian plutonism in Iran [14]. Hassanzadeh et al.

[14] by using TIMS zircon U-Pb geochronology found

two astonishingly different age for Doran granite (2.8 ±

0.1Ma and 567 ± 19Ma). Using cathodoluminescence

Figure 5. Photomicrograph of biotite-chlorite-muscovite

schist showing a) S1 schistosity b) S2 schistosity

c) S2 and S3 schistosities.

Vol. 23 No. 2 Spring 2012 Ghadimi et al. J. Sci. I. R. Iran

152

Figure 6. Mesoscopic orientation data for a) S1 schistosity and L1 lineation b) F1 fold c) S2

schistosity and L2 lineation d) F2 fold e) S3 schistosity and L3 lineation f) F3 Fold.

method, they revealed the inherited cores in some of the

zircons. Considering similar granite in Soltanieh belt

such as Sarvejahan and Moghanlou, they believe to the

existence of Neoproterozoic plutonism [14].

2-Deformational Phases

The main study area is along a northwest-southeast

traverse stretching from Chavarzagh village towards the

Rayhan village (Fig. 3). Field studies have focused on

mesoscopic structures and several discrete deformation

stages are recognized.

2-1- D1 Deformation

The main structures formed during deformation stage

of D1 are a S1 schistosity and associated stretching


153

lineation (L1) (Figs. 4a, 4b and 5a). The main L1

forming minerals are chlorite and muscovite, which are

aligned on the S1 plane (Fig. 5a). S1 and L1 are dominant

in the northwestern part of the study area. The

orientation data of S1 and L1 are plotted in Figure 6a.

Mesoscopic folds formed during D1 (shown as F1) are

dominantly found in the northwestern part of the study

area. F1 folds are generally tight with an orientation

trend as shown in Figure 6b. These folds are regarded to

be formed during D1 because: (i) the folds have flat

lying axial planes and N-S trending axes, and the

orientations of these structures are the same as those of

S1 and L1 (Figs. 6a and 6b) and (ii) a newly formed

schistosity (S1) is commonly observed subparallel to the

axial planes of the folds and stretching lineation parallel

to the fold axes (Figs. 4a, 4b and 5a). It is widely

recognized irrespective of the original orientation that

folds associated with high strain will tend to have their

axes parallel to the maximum finite stretching direction

and their axial planes will be parallel to the new

schistosity (e.g. [40]).

2-2- D2 Deformation

D2 is a post-D1 phase of deformation recognized most

readily by kink-like mesoscopic folds that fold S1 (Figs.

4a and 4b). This type of fold, generally on a centimeter

scale is termed F2. In this area, most of the F2

crenulations (with wavelength about 1 Cm) on the S1

plane are oblique to L1 and L1 is clearly folded by F2

crenulation (Figs. 4a and 4b). The orientation data of F2,

S2 and L2 are plotted in Figures 6c and 6d and show that

the orientation data of F2 are the same as those for S2

and L2 structures, therefore it can be inferred that both

were formed during D2 deformation. Chlorite,

muscovite, biotite and staurolite are mostly found in the

limbs while quartz and albite can be recognized in the

hinges of crenulations (Fig. 5b).

2-3- D3 Deformation

The main structures formed during this deformation

are strongly developed crenulation cleavage (S3) and

associated stretching lineation (L3) (Fig. 4c). The main

L3 forming minerals are biotite, chlorite and muscovite

and their alignment on the S3 plane can be observed

(Fig. 5c). This deformation produce the dominant

structures in the study area and developed by folding the

previous foliations. Although the effect of the D3 can be

recognized in all of the study area but dominantly can

be seen in southeastern part which diminished other

traces of older deformations. Mesoscopic to

macroscopic folds formed during D3 (shown as F3) are

also frequently found in the southeastern part with an

orientation shown in Figure 6f. The orientation data

show that newly formed schistosity (S3) is commonly

observed subparallel to the axial planes of folds and the

related stretching lineation parallel to the fold axes

(Figs. 6f and 6e). S3 and L3 were defined by biotite,

muscovite and chlorite crystals oriented parallel to the

axes of the crenulations (Fig. 5c). Sometimes S3 is

identified by spaced foliation defined by mica-rich

bands and quartz-feldspar bands both in mesoscopic and

microscopic scales.

3- Petrography

From northwest (Chavarzagh village) towards

southeast (Rayhan village) of the study area, three

mineral zones by increasing metamorphic grade have

been recognized.

Chlorite zone: Rocks of the chlorite zone are

phyllite, chlorite schist, and muscovite chlorite schist.

Mineral assemblage of this zone consists of quartz,

muscovite, chlorite, albite ± orthoclase (Table 1).

Biotite zone: The main observed metamorphic rocks

of the study area belong to the biotite zone. The main

rock type of this zone includes muscovite-chlorite-

biotite schist. Mineral assemblage of the biotite zone

contains biotite, albite, chlorite, quartz, muscovite ±

orthoclase. (Table 1).

Staurolite zone: The first occurrence of staurolite is

in the staurolite schist near the Rayhan village in SE of

the study area. The mineral assemblage of staurolite

schist is staurolite, biotite, muscovite, chlorite,

plagioclase and quartz (Table 1). Other metamorphic

rocks such as talc schist and quartzite are also observed

as intercalations within pelitic schist.

Muscovite is present in all of the rocks examined but

its size, modal percentage and chemical composition

vary from chlorite to staurolite zones and also in

different textural domains. In the chlorite zone, fine

grain (usually less than 0.1mm) muscovite can be found

within slaty cleavage which it can be termed sercitie

instead of muscovite. But in biotite and staurolite zones,

muscovite up to 0.3mm long has developed in the rocks.

Its modal percentage decreases with increasing

metamorphic grade. Muscovite is a major constituent

mineral of D1, D2 and D3 phases of deformations,

forming L1 alignment on the S1 plane and L2 on the

limbs of F2 crenulation and L3 associated stretching

lineation developed in D3.

Chlorite is present in all of the examined rocks but is

a major constituent minerals especially in the rocks of

the chlorite and biotite zones. It is also occurred in

different textural domains. In the chlorite zone, it is

usually less than 0.1mm in size and is recognized in the

slaty cleavage of the constituent rocks of this zone but


154

Table 1. Parageneses of the main studied samples. (+) and (-) indicate presence or absence of the minerals respectively

Sample No. SZ42 SZ41 BZ23 BZ22 BZ21 CZ13 CZ12 CZ 10

Muscovite + + + + + + + +

Chlorite + + + + + + + +

Biotite + + + + + − − −

Staurolite + + − − − − − −

Albite + + + + + + + +

Quartz + + + + + + + +

Alkalifeldspar − − − − − + + +

Table 2. Representative analyses of the main mineral groups

Mineral name Staurolite Albite Chlorite Biotite Muscovite

Metamorphic zone

St

St Bt Bt

St Bt Bt

St Bt Bt

St Bt Bt

Deformation phase

D3 D3 D3 D2

D3 D3 D2 D3 D3 D2

D3 D3 D2

Sample No

SZ42

SZ42 BZ23 BZ22

SZ42 BZ23 BZ22

SZ42 BZ23 BZ22

SZ42 BZ23 BZ22

Point No

221

220 112 111

211 102 101

215 103 107

210 109 105

SiO2 25.06

63.3 64.01 65.93

26.11 26 25.37

34.4 33.82 34.7

46.82 49.01 50.12

TiO2

0.2

- - -

0.33 0.3 0.2

1.01 1.9 1.03

0.2 0.15 0.1

Al2O3 54.62

24 22.2 21.62

23.12 25.01 24.12

19.21 19.62 18.52

34.8 33.21 32.19

FeO

13.84

- - -

24.22 19.02 21.02

21.04 19.7 22.4

1.14 1.04 0.89

MgO

1.41

- - -

14.67 18.01 17.51

10.61 8.21 6.83

0.48 0.51 0.71

CaO

0.2

3.01 2.12 1.12

0.01 0.03 0.02

0.1 0.1 0.1

0.04 0.03 0.05

Na2O

0.1

9.5 10.9 11.5

- - -

0.1 0.12 0.1

0.1 0.2 0.18

K2O

-

0.07 0.08 0.1

0.05 0.05 0.06

9.12 9.01 8.89

8.85 8.5 8.23

Total

95.43

99.88 99.31 100.3

88.51 88.42 88.3

95.59 92.48 92.58

92.43 92.66 92.48

O=22

O=8

O=14

O=11 O=11

O=11

Si

7.16

2.794 2.842 2.907

2.692 2.637 2.584

2.614 2.666 2.756

3.16 3.28 3.349

Ti

0.043

- - -

0.026 0.02 0.015

0.058 0.11 0.062

0.01 0.008 0.005

Altet

-

1.248 1.162 1.093

1.308 1.362 1.416

1.386 1.334 1.244

0.84 0.72 0.651

Aloct

18.4

- - -

1.503 1.627 1.481

0.335 0.486 0.49

1.929 1.9 1.885

Fe2+

3.307

- - -

2.089 1.613 1.791

1.118 1.298 1.488

0.052 0.06 0.05

Fe3+

-

- - -

- - -

0.197 - -

0.011 - -

Mg

0.6

- - -

2.255 2.725 2.658

1.202 0.947 0.808

0.048 0.05 0.071

Ca

0.061

0.142 0.101 0.053

0.001 0.003 0.002

0.008 0.009 0.009

0.003 0.002 0.004

Na

0.055

0.811 0.938 0.983

- - -

0.015 0.02 0.015

0.013 0.03 0.023

K

-

0.002 0.005 0.006

0.007 0.006 0.008

0.885 0.9 0.902

0.763 0.73 0.702

Total

29.62

4.997 5.048 5.042

9.881 9.993 9.955

7.818 7.77 7.774

6.829 6.78 6.740

Xmu

-

- - -

- - -

- - -

0.84 0.72 0.651

Xcel -

- - -

- - -

- - -

0.048 0.05 0.071

Xphl

-

- - -

- - -

0.4 0.312 0.269

- - -

Xann -

- - -

- - -

0.4 0.2 0.241

- - -

Xames -

- - -

0.44 0.391 0.334

- - -

- - -

Xclin

-

- - -

0.275 0.341 0.368

- - -

- - -

Xab -

0.811 0.938 0.983

- - -

- - -

- - -

Xan -

0.187 0.057 0.011

- - -

- - -

- - -


155

in the biotite zone it occasionally reaches up to 0.5mm

long can be seen as alignted parallel to the S3

schistosity. It’s modal percentage decreases towards the

staurolite zone and in this zone it is minor constituent

minerals. Chlorite is also present in D1, D2 and D3

phases of deformation.

Biotite was found in pelitic schists of the biotite and

staurolite zones and it is one of the major constituent

minerals of these zones. It is also occurred in two

microstructural domains of D2 and D3.

Albite is present in the rocks of chlorite, biotite and

staurolite zones. It’s size and chemical composition

changes with increasing grade of metamorphism from

chlorite zone towards the staurolite zone. In the chlorite

zone it is usually very fine and is very difficult to

distinguish it from quartz. Albite is also occurred in two

microstructural domains of D2 and D3.

4- Mineral Chemistry

Mineral analysis was preformed with a Cameca

SX100 electron probe microanalyzer (10 KV, 10 nA)

using albite (Na), orthoclase (Al, K), anorthite (Ca),

diopside (Mg, Si), MnTiO3 (Mn, Ti) and Fe2O3 (Fe) as

standards in KMA research group in Malaysia. The

structural formulae, the Fe3+

-iron and end-member

activities of the minerals were recalculated with the Ax

program of Holland and Powell (www.esc.com.ac.uk/

pub/minp/AX). Mineral abbreviations are from Holland

and Powell [16].

4-1- Muscovite

Muscovite analyses are listed in Table 2. The

number of cations is calculated on the basis of 11

oxygens. The main constituent end-member is

muscovite and celladonite and paragonite being the

other end-members. Comparison of the muscovite

compositions from the biotite and staurolite zones

shows that muscovite in staurolite zone contains more

muscovite end-member (biotite zone muscovite

composition: Xmu = 0.718, Xcel = 0.051; staurolite zone

muscovite composition: Xmu = 0.84, Xcel = 0.048) (Table

2). Reduction in the phengite substitution of muscovite

with metamorphic grade has been described by several

authors in different low to medium pressure

metamorphic terrains [39, 9]. There are no distinct

compositional differences between those lying parallel

to the S1 schistosity and those oblique to S1 on S2

schistosity. Muscovite of D3 deformation is richer in

muscovite end-member and slightly depleted in

celladonite component (D2-muscovite composition: Xmu

= 0.651, Xcel = 0.07; D3-muscovite composition: Xmu =

0.718, Xcel = 0.051) (Table 2).

4-2- Chlorite

Representative analyses of chlorite are tabulated in

Table 2. The number of cations is calculated on the

basis of 14 oxygen. According to Table 2, chlorite is

composed of two main end-members of clinochlore and

amesite. The mole fractions of other component are

negligible. Comparing the chemical composition of

chlorite from the biotite and staurolite zones, it can be

inferred that with increasing metamorphic grade from

the biotite zone towards staurolite zone, the amesite

component of chlorite increases while clinochlore

component decreases (biotite zone chlorite composition:

Xames = 0.391, Xclin = 0.341; Staurolite zone chlorite

composition: Xames = 0.44, Xclin = 0.275 (Table 2).

Regarding differences of chemical compositions

between chlorite lying parallel to the S1 and S2, distinct

compositional differences cannot be observed.

Comparing chemical compositions of chlorites formed

during D2 and D3 deformations show that D2-chlorite is

richer in clinochlore and poorer in amesite end-

members (D2-chlorite composition: Xclin = 0.368, Xames

= 0.334; D3-chlorite composition: Xclin = 0.341, Xames =

0.391). (Table 2, Fig. 7b).

4-3- Biotite

Microprobe analyses of biotite are provided in Table

2. The number of cations is calculated on the basis of 11

oxygen. On the average, the phlogopite and annite end-

members made up to 60 percentage of biotite

composition and the remaining is mostly eastonite

component (Table 2). From the biotite zone towards the

staurolite zone, the phlogopite end-member increases

(biotite zone biotite composition: Xphl = 0.312, Xann =

0.20; staurolite zone biotite composition: Xphl = 0.4,

Figure 7. Stable intersection involving muscovite, paragonite,

clinochlore, phlogopite, eastonite, albite, quartz and

H2O for M2 assemblage.


156

Table 3. The stability periods for minerals and the relationship between metamorphism, mineral crystallization, mineral composition

and deformational phases

Deformational Phases

Mineral D1 D2 D3 Microstructural domains Mineral compositin

name Metamorphic stages

M1 M1 M2

Muscovite L1 alignment on the S1 plane Xmu=0.651 , Xcel=0.07

Muscovite Limbs of F2 crenulation Xmu=0.651 , Xcel=0.07

Muscovite L3 stretching lineation on S3 plane Xmu=0.718 , Xcel=0.051

Chlorite L1 alignment on the S1 plane Xclin=0.368 , Xames=0.334

Chlorite Limbs of F2 crenulation Xclin=0.368 , Xames=0.334

Chlorite L3 stretching lineation on S3 plane Xclin=0.341 , Xames=0.391

Biotite Limbs of F2 crenulation Xphl=0.269 , Xann=0.241

Biotite L3 stretching lineation on S3 plane Xphl=0.312 , Xann=0.20

Staurolite Limbs of F2 crenulation Fe-Mg Staurolite

Albite Hinges of F2 crenulation Xab=0.983 , Xan=0.011

Albite L3 stretching lineation on S3 plane Xab=0.983 , Xan=0.057

Quartz

Xann = 0.4). Biotite from D3 deformation contains more

phlogopite and slightly less annite end–member (D2–

biotite composition: Xphl = 0.269, Xann = 0.241; D3-

biotite composition: Xphl = 0.312, Xann = 0.20) (Table 2).

4-4- Albite

The major constituent end-member of albite is albite

and anorthite being the second constituent (Table 2). It’s

chemical composition changes from the biotite zone

towards the staurolite zone in which it becomes richer in

anorthite and poorer in albite end-members (Chemical

composition of albite from biotite zone: Xab = 0.938, Xan

= 0.057; chemical composition of albite from staurolite

zone: Xab = 0.811, Xan = 0.187) (Table 2). Also, clear

differences between albite in S3 schistosity and those of

S2 schistosity can be seen. Albite forming in D3

deformation is richer in anorthite and poorer in albite

components (chemical composition of albite of D3-

deformation: Xab=0.938, Xan=0.057; chemical

composition of albite of D2-deformation: Xab=0.983,

Xan=0.011) (Table 2).

4-5- Staurolite

Staurolite is a porphyroblast phase and only occurs

in staurolite zone in the southeast of the area where only

S2 schistosity can be seen. Commonly it is euhedral and

chemically homogenous. The average chemical

composition of staurolite is tabulated in Table 2.

5- Thermobarometry

Considering the relationship between deformational

Table 4. 6 stable reactions (a) and 4 independent set of

reactions (b) obtained for M� assemblage containing white

mica, chlorite, biotite, plagioclase, quartz and fluid

phases in the K2O-Al2O3–SiO2–MgO–FeO–Na2O–CaO–H2O

(KNCMFASH) system

a)

[1] 5mu+3clin+8ab = 8pa+5phl+9q+4H2O

[2] 14pa+11phl = 2mu+3clin+9east+14ab+2H2O

[3] 6pa+5phl = clin+5east+6ab+2q+2H2O

[4] mu+2pa+2phl = 3east+2ab+3q+2H2O

[5] 3mu+clin+phl = 4east+7q+4H2O

[6] 5mu+2clin+2ab = 2pa+5east+11q+6H2O

b)

[7] Clin+east = ames+phl

[8] 2pa+3cel = 2mu+phl+2ab+2H2O+3q

[9] 14pa+9ames+20ann=2mu+12daph+18east+14ab+2H2O

[10]8pa+7ames+15ann+2q = 2cel+9daph+13east+8ab


157

phases, microstructural domains and mineral

compositions, two metamorphic stages can be

recognized: (1) a higher pressure metamorphic event

(M1) that is produced during the D1 and (2) a lower

pressure and higher temperature metamorphism (M2)

that has resulted from D3 episode of deformation (Table

3). D1 recognized by a S1 schistosity and associated

stretching lineation (L1) produced by generally tight

folds of F1 and D2 recognized by a S2 and L2 structures

originated by F2 crenulation. Altough D1 and D2 are

vividly two different deformational phases but their

mineral compositions are similar, therefore, they have

been attributed to a single metamorphic stage of M1

(Table 3). The first metamorphic stage (M1) is

characterized by mineral assemblage of muscovite,

chlorite, biotite, staurolite and quartz forming the S1 and

S2 schistosities. The second metamorphic stage (M2)

can be recognized by mineral assemblage like

muscovite, chlorite, biotite, albite and quartz forming L3

on the S3 plane that produced during D3 episode of

deformational phase.

Selected minerals involved in the same

microstructural domain and in close contact are

regarded to have crystallized coevally in thermo-

dynamic equilibrium. For calculations, we used the

internally consistent thermodynamic data set of Holland

and Powel [16] and the program TERMOCALC V3.21

[15, 16]. Accuracy on the P-T estimates, estimated by

the error ellipse parameters is typically of the order of

10-30°C for temperature and 1-2kbars for pressure.

Some end-members presented only in small amounts

such as magnesio-staurolite for staurolite which produce

a large uncertainty on the results were removed but in

some cases such as daphinite in chlorite was not omitted

because of its importance for finding metamorphic

reactions. Pressures and temperatures during

metamorphism were estimated using crossing reactions

curves of independent reactions obtained from average

P-T mode incorporated in TERMOCALC V3.21. To

ensure that the estimated P-T is not an artifact, stable

invariant points and univariant reactions were obtained

using Schreinemakers’ analysis. The P-T positions of

the invariant point and univariant equilibria have been

determined using THERMOCALC V3.2.1. For the

construction, the fluid phase was assumed to be in

excess and water activity was assumed to be 1.0 as

already inferred from the observed metamorphic

assemblage. For M� assemblage, containing white mica,

chlorite, biotite, plagioclase, quartz and water, the

system K2O-Al2O3–SiO2–MgO–FeO–Na2O–CaO–H2O

(KNCMFASH) with phase components including

muscovite, paragonite, celladonite, clinochlore,

daphinite, amesite, phlogopite, annite, eastonite, albite,

anorthite, quartz and H�O was considered. Doing

Schreinemakers’ analysis for the system gives 31

univariant reactions and 11 intersections in which only

one intersection is stable (Fig. 7). The stable intersection

involving muscovite, paragonite, clinochlore,

Figure 8. Cross cutting of independent

reactions for M2 assemblage.

Figure 9. Stable intersections for M1 assemblage containing

phases of white mica, chlorite, biotite, plagioclase, staurolite,

quartz and fluid. a) Stable intersection containing mu, cel, clin,

daph, ann, fst, q and H2O. b) Stable intersection containing

mu, cel, clin, phl, ann, fst, q and H2O. c) Stable intersection

containing mu, clin, ames, phl, ann, east, fst, and H�o d)

Stable intersection containing mu, clin, ames,

phl, ann, east, fst and H2O.


phlogopite, eastonite, albite, quartz and H

ames, daph, ann] contains 6 stable reactions

(Table 4a). An invariant point (at P=5.6kbar

(Fig. 7) has been calculated for the mentioned system

in which activities of the phases adjusted for one as a

model system. Independent set of reactions has been

calculated using average P-T mode in THERMOCALC

(Table 4b) (Fig. 8). They cross cut aaround

670°C (Fig. 8). Both calculations vividly show that the

first stage of metamorphism was high-temperature and

relatively low pressure.

M1 assemblage contains phases of white mica,

chlorite, biotite, plagioclase, staurolite, quartz and water

in the system of K2O–Al2O3–SiO2–MgO

CaO–H2O (KNCMFASH) with end

components including muscovite, paragonite,

celladonite, clinochlore ,daphinite, amesite, phlogopit

annite, eastonite, Fe-staurolite, albite, anorthite, quartz

and H2O. Clearly, the chemical compositions and modal

percentages of the M�-stage differ from

Schrinemakers’ analysis of the system indicates four

stable intersections which except one of them cross cut

at P=8-9kbar, T=561-589°C (Fig. 9). They include

stable intersections [pa, ames, phl, east, ab] involving

muscovite, celladonite, clinochlore, daphinite, annite,

Fe-staurolite, quartz, H2O; [pa, ames, daph, east, ab]

involving muscovite, celladonite, clinochlore,

phlogopite, annite, Fe-staurolite, quartz, H

daph, ab, q] involving muscovite, annite, eastonite, Fe

staurolite, H2O and [pa, cel, ames, daph, ab] involving

muscovite, clinochlore, phlogopite, annite, eastonite,

Fe-staurolite, quartz, H2O (Fig. 9, Table

Independent set of reactions obtained from average P

calculation in THERMOCALC gives an average

temperature of 600°C and pressure of 8kbar pressure

(Fig. 10) (Table 5e).

Results and Discussion

The commonly accepted Neoproterozoic

paleogeographic reconstruction of the continents, place

the Iranian terranes along the Paleotethyan margin of

Gondwana supercontinent between the Zagros margin

of Arabia and northwestern margin of the Indian plate

(e.g. [33, 25, 14]). It usually discussed that the Late

Neoproterozoic-Early Cambrian magmatism and

sedimentation in Iran is the result of crustal extension

associated with continental drift which was

accompanied by emplacement of alkaline magmatism

[5,27,2]. It is believed that the vast carbonate

deposits of the Zagros mountains (Hormuz Formation)

and their correlative rocks in the region (e.g. Salt Range

in Pakistan) are deposited in the rift basins associated

Ghadimi et al.

158

phlogopite, eastonite, albite, quartz and H2O or [cel,

stable reactions (Fig. 7)

kbar, T=710°C)

has been calculated for the mentioned system,

in which activities of the phases adjusted for one as a

Independent set of reactions has been

T mode in THERMOCALC

They cross cut aaround 5kbar and

Both calculations vividly show that the

temperature and

of white mica,

chlorite, biotite, plagioclase, staurolite, quartz and water

MgO-FeO-Na2O–

O (KNCMFASH) with end-member

components including muscovite, paragonite,

celladonite, clinochlore ,daphinite, amesite, phlogopite,

staurolite, albite, anorthite, quartz

. Clearly, the chemical compositions and modal

differ from M1-stage.

Schrinemakers’ analysis of the system indicates four

e of them cross cut

They include

stable intersections [pa, ames, phl, east, ab] involving

muscovite, celladonite, clinochlore, daphinite, annite,

O; [pa, ames, daph, east, ab]

, celladonite, clinochlore,

staurolite, quartz, H2O; [pa, cel,

daph, ab, q] involving muscovite, annite, eastonite, Fe-

O and [pa, cel, ames, daph, ab] involving

muscovite, clinochlore, phlogopite, annite, eastonite,

Table 5a,b,c,d).

Independent set of reactions obtained from average P-T

calculation in THERMOCALC gives an average

kbar pressure

Neoproterozoic

the continents, place

the Iranian terranes along the Paleotethyan margin of

Gondwana supercontinent between the Zagros margin

of Arabia and northwestern margin of the Indian plate

It usually discussed that the Late

Early Cambrian magmatism and

sedimentation in Iran is the result of crustal extension

associated with continental drift which was

accompanied by emplacement of alkaline magmatism

believed that the vast carbonate-evaporate

deposits of the Zagros mountains (Hormuz Formation)

and their correlative rocks in the region (e.g. Salt Range

in Pakistan) are deposited in the rift basins associated

with the post-collisional, transcurrent faul

after the Pan-African orogeny [5,20].

Talbot and Alavi [37] proposed an aborted rift tectonic

model involving Late Neoproterozoic

crust that underwent extension, but failed to drift apart,

along the margin of the Gondwana.

the early Cambrian magmatism to the extensional rifting

of the crust and asthenospheric upwelling.

alternative scenario, Ramezani and Tucker

Neoproterozoic to Early Cambrian orogenic activity in

an active continental margin environment.

geochronological and geochemical observation in

Figure 10. Cross cutting of independent

reactions for M1 assemblage.

Figure 11. P-T path obtained for the studied metamorphic

rocks. 1 and 2 are paths of Franciscan and Alpine type of

subduction zone metamorphism (Spear

paths for the lower and upper plates of Acadian terrain of New

England provided for comparison (Spear et al

5 is the P-T path obtained from this study.

J. Sci. I. R. Iran

collisional, transcurrent faults produced

]. On the other hand,

proposed an aborted rift tectonic

model involving Late Neoproterozoic-Early Cambrian

crust that underwent extension, but failed to drift apart,

They also attributed

the early Cambrian magmatism to the extensional rifting

of the crust and asthenospheric upwelling. As an

Ramezani and Tucker [25] using

Neoproterozoic to Early Cambrian orogenic activity in

continental margin environment. They believe

geochronological and geochemical observation in

Cross cutting of independent

assemblage.

T path obtained for the studied metamorphic

are paths of Franciscan and Alpine type of

Spear [30]). 3 and 4 are P-T

paths for the lower and upper plates of Acadian terrain of New

Spear et al. [31]).

om this study.


159

central Iran proposed a major episode of Latest that

metamorphic complex in central Iran indicate a Late

Neoproterozoic-Early Cambrian metamorphism in an

orogenic setting and that the Early Cambrian granites as

well as the volcanic rhyolite-dacite associations

demonstrably originated in active margin environment

and do not show alkaline affinities attributable to intra-

plate magmatism. They also proposed that Early

Cambrian evaporate facies were developed in

extentional, back-arc environment [25]. This hypothesis

was also supported by other researchers (e.g. [14,17]).

In the subduction setting such as Japan, Turkey, the

Cyclades (Greece), New Zealand [9, 10, 39] the P-T

path shows a generally clockwise loop where the high

pressure-low temperature subduction zone metamor-

phism is followed by decompression, often with

substantial heating. The metamorphic consequence of

this P-T path is extensive greenschist or amphibolites

facies overprint on the early blueschist and eclogite

facies assemblages [10]. On the other hand, the P-T path

from continental collision settings such as Acadian

orogeny in central New England, U.S.A. [30], the early

part of the path is nearly isobaric heating passing

through the andalusite field and is interpreted to result

from regional contact metamorphism. The thermal peak

was followed by a period of loading followed by

cooling, which is interpreted as representing the nappe

stage of deformation. The path for the lower plates also

shows an early low pressure part in the andalusite field

followed by an increase in pressure followed by minor

heating or cooling [30] (Fig. 11). Crustal temperature

can also increase during lithospheric extension [22],

leading to incipient rifting with or without magmatic

underplating. Alkaline magmatism can itself be

considered as a product of such rifting. The

subsequence loading must then be an expression of

shortening that immediately followed extension, with

the time interval between the two processes being small

enough for heat to be inherited by the compressional

phase from the preceding extensional phase during

tectonic regime inversion [28,12]. Such thermally

softened extensional zones are vulnerable to later

thickening during orogenesis [38]. It was shown in this

study that Late Neoproterozoic-Early Cambrian schists

in southwest of Zanjan underwent two stages of

metamorphism (M1 and M2). The average P-T

calculated for independent set of reactions and

Schrinemakers, analysis for M2 and M1 stages of

metamorphism were 5.3Kbar and 690°C and 8.5Kbar

and 580°C respectively (Fig. 11). Thus, the inferred

pressure-temperature path represents a clockwise P-T

path from intermediate pressure Barrovian type

metamorphism (M1) towards low pressure-high

Table 5. Stable reactions (a, b, c, d) and independent set of

reactions (e) obtained for M� assemblage containing white

mica, chlorite, biotite, plagioclase, staurolite, quartz and fluid

phases in the K2O-Al2O3–SiO2–MgO–FeO–Na2O–CaO–H2O

(KNCMFASH) system. a) Stable reactions containing mu, cel,

clin, phl, ann, fst, q and H2O. b) Stable reactions containing

mu, cel, clin, phl, ann, fst, q and H2O. c) Stable reactions

containing mu, clin, ames, phl, ann, east, fst, and H2O. d)

Stable reactions containing mu, clin, ames, phl, ann, east, fst

and H2O. e) Independent set of reactions for M1 assemblage

containing white mica, chlorite, biotite, plagioclase, staurolite,

quartz and fluid phases

a)

[11] 400mu+33clin+197daph=165cel+235ann+70fst+780H2O

[12] 205mu+41clin+24daph+25q=205cel+30fst+200H2O

[13] 131mu+31clin+24ann+197q=155cel+18fst+88H2O

[14] 20cel+9daph=5mu+4clin+15ann+35q+20H2O

[15] 205cel+131daph=41clin+205ann+10fst+400q+340H2O

[16] 41mu+31daph=41ann+8fst+33q+108H2O

b)

[17] 240mu+138clin+56ann=99cel+197phl+42fst+468H2O

[18] 131mu+31clin+24ann+197q=155cel+18fst+88H2O

[20] 4cel+clin=mu+3phl+7q+4H2O

[21] 123cel+54clin+8ann=131phl+6fst+240q+204H2O

[22] 123mu+93clin+32ann=155phl+24fst+99q+324H2O

c)

[23] 141mu+138clin+56ann+99east=296phl+42fst+468H2O

[24] Clin+east=ames+phl

[25] 141mu+296ames+56ann=158clin+197east+42fst+468H2O

[26] 141mu+138ames+56ann=158phl+39east+42fst+468H2O

[27] 141mu+39clin+99ames+56ann=197phl+42fst+468H2O

d)

[28] 13clin+8ann+41east+47q=49phl+6fst+40H2O

[29] 39mu+62phl+6fst=8ann+93east+138q+12H2O

[30] 3mu+clin+phl=4east+7q+4H2O

[31] 147mu+62clin+8ann=155east+6fst+296q+236H2O

[32] 123mu+93clin+32ann=155phl+24fst+99q+324H2O

[33] 141mu+138clin+56ann+99east=296phl+42fst+468H2O

e)

[34] Clin+east=ames+phl

[35] 85mu+8daph+50plh+195q=135cel+3clin+10fst

[36] 85mu+27clin+50ann+195q=135cel+22daph+10fst

[37] 35mu+8daph+50east+195q=85cel+3clin+10fst

[38] 82mu+8daph+41phl+174q=123cel+10fst+12H2O

[39] 94mu+8daph+47phl+12ab+192q=12pa+141cel+10fst


160

temperature Buchan type metamorphism (M2) (Fig. 11).

The obtained clockwise P-T path is characteristic of the

metamorphic evolution of orogenic belts whose

formation involved tectonic thickening of the crust [30,

31]. The thermobarometric estimates show that the

studied rocks were at a depth of 28-31 km when they

experienced their peak metamorphic temperature of

561-600°C. This reflects a mean geothermal gradient of

about 30°C/km that can be considered as an indicative

for collision tectonics [30]. The convergence and crustal

thickening implied by this intermediate pressure event

was followed by low pressure-high temperature

metamorphism (M2). The deduced P-T path shows that

a pressure decrease of 3-4 Kbar was responsible for the

retrograde P-T path since it is accompanied by an

almost 100°C temperature increase. The increasing of

temperature may show that exhumation was

accompanied by magmatic activity such as Doran

granite in the studied area. Compare with equivalent

units from central Iran [30] suggests that metamorphic

evolution of Late Neoproterozoic-Early Cambrian

schists in southwest of Zanjan took place during the

Pan-African orogeny.

Acknowledgements

The University of Zanjan supported this study; this

support is gratefully acknowledged. The authors are

deeply grateful to anonymous reviewer for constructive

comments and English editing efforts.

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